Software-defined networking (SDN) is an approach to computer networking that allows network administrators to manage network services through abstraction of lower-level functionality. SDN is meant to address the fact that the static architecture of traditional networks doesn't support the dynamic, scalable computing and storage needs of more modern computing environments such as data centers. This is done by decoupling or disassociating the system that makes decisions about where traffic is sent (the control plane) from the underlying systems that forward traffic to the selected destination (the data plane).

SDN was commonly associated with the OpenFlow protocol (for remote communication with network plane elements for the purpose of determining the path of network packets across network switches) since the latter's emergence in 2011. Since 2012,[1][2] however, many companies have moved away from OpenFlow, and have embraced different techniques. These include Cisco Systems' Open Network Environment and Nicira's network virtualization platform.

One of the first SDN projects was AT&T's GeoPlex.[7]AT&T Labs Geoplex project members Michah Lerner, George Vanecek, Nino Vidovic, and Dado Vrsalovic leveraged the network APIs and dynamic aspects of the Java language as a means to implement middleware networks. "GeoPlex is not an operating system, nor does it attempt to compete with one. It is networking middleware that uses one or more operating systems running on computers, connected to the Internet. GeoPlex is a service platform that manages networks and on-line services. GeoPlex maps all of the IP network activities into one or more services."[8]

GeoPlex did not concern itself with operating systems running on networking hardware switches, and routers. AT&T wanted a "soft switch" that could reconfigure physical switches in the network and load them with new services from an operations support system (OSS). However, when provisioning services GeoPlex could not reach deeply into the physical devices to perform reconfiguration. The operating systems running on networked devices in the physical network therefore became a barrier to early SDN-like service delivery.

In 1998, Mark Medovich, a senior scientist of Sun Microsystems and Javasoft, left Sun to launch a Silicon Valley soft switch startup WebSprocket. Medovich designed a new network operating system, and an object oriented structured runtime model that could be modified by a networked compiler and class loader in real time. With this approach, applications could be written with Java threads that inherited WebSprocket kernel, network, and device classes and later modified by the networked compiler/class-loader. WebSprocket's platform was designed such that devices had the ability to instantiate network stack(s), interfaces, and protocols as multiple threads.[9]

In July 2000, WebSprocket released VMFoundry, the Java to bare metal structured runtime compiler, and VMServer, a networked device compiler/classloader application server.[10] Custom networked devices were preloaded with images created by VMFoundry then deployed on the network and connected to VMServer via UDP or TCP services plane, which could proactively or reactively load or extend network protocol methods and classes on the target system. WebSprocket's version of SDN, therefore was not confined to a set of limited actions managed by an SDN controller. Rather, WebSprocket's "control plane" contained code that could change, override, extend, or enhance Network protocols on operating networked systems.[11] Bill Yount (Stanford University Network) visited WebSprocket's Sunnyvale lab to see a demonstration and expressed great enthusiasm about the entire concept, especially the VMServer (SDN Controller) and prophetically stated SDN (WebSprocket) as "10 years ahead of its time". In Summer of 2000, Ericsson's advanced network research engineers saw an immediate need and visited WebSprocket to design and architect features of a next generation soft switch thus taking first steps to build the world's first commercial soft switch.

Sometime during 2000, the Gartner Group introduced the "Supranet", the fusion of the physical and the digital (virtual) worlds as "internet of things," and by October 2000 the Gartner Group selected WebSprocket as one of the top emerging technologies.[12]

In early 2001, Ericsson and WebSprocket entered into a license contract to create the first commercial soft switch. An international consortium was formed to develop standards for the "Supranet". In March 2001, Kurt Dewitt, Supranet Consortium Chairman and Business Development Director for Ericsson's Data Broadband and Optical Networks Division, announced the selection of WebSprocket as the enabling technology of the Supranet Transaction Server (STS), a comprehensive framework to deliver any networked service.[13]

In April and May 2001, Ohio State University and OARnet, collaboratively ran the first SDN test and developed the first practical SDN use case for Internet2. After successful completion of tests, OARnet issued the following statement on May 8, 2001:

"We have witnessed the successful first step to the fulfillment of smart, interoperable networks through the deployment of Supranet Transaction Server. A technology first was accomplished as a new set of instructions was dynamically transmitted across the network, changing the behavior of the requesting computer. There was no need to take down any part of the system and there was no interruption of service. Our testing will continue and we anticipate further advancement of the next generation Internet through our partnership with Websprocket" – Pankaj Shah (Managing Director, OARnet)[14]

The telecom market deflated in 2001 and Ericsson's soft switch development program came to an end, thus stalling the only known commercial SDN soft switch R&D effort at that time.

Software-defined networking was continued with work done in 2003 by Bob Burke and Zac Carman developing the Content Delivery Control Network patent application that eventually was issued as two US patents: 8,122,128[15] and 8,799,468.[16] In this seminal inception, SDN, named service preference architecture (SPA) in their patent, was described as a collection of network embedded computing techniques used to control the operation of Network Elements, namely content servers, routers, switches and gateways, with the objective being to safeguard content from theft (P2P) or unwanted interception and to efficiently deliver content for paid services. CableLabs later specified Digital Cable and CableCARD using what we now know as SDN, which debuted in 2007. SDN was again moved ahead in work done at UC Berkeley and Stanford University around 2008.[17]

At the 2014 Interop and Tech Field Day, software-defined networking was demonstrated by Avaya using shortest path bridging and OpenStack as an automated campus, extending automation from the data center to the end device, removing manual provisioning from service delivery.[18][19]

By 2016, some in the industry thought that SDN had become a meaningless marketing term.[20]

Software-defined networking (SDN) is an architecture purporting to be dynamic, manageable, cost-effective, and adaptable, seeking to be suitable for the high-bandwidth, dynamic nature of today's applications. SDN architectures decouple network control and forwarding functions, enabling network control to become directly programmable and the underlying infrastructure to be abstracted from applications and network services.[21]

The OpenFlow protocol can be used in SDN technologies. The SDN architecture is:

Directly programmable: Network control is directly programmable because it is decoupled from forwarding functions.

Centrally managed: Network intelligence is (logically) centralized in software-based SDN controllers that maintain a global view of the network, which appears to applications and policy engines as a single, logical switch.

Open standards-based and vendor-neutral: When implemented through open standards, SDN simplifies network design and operation because instructions are provided by SDN controllers instead of multiple, vendor-specific devices and protocols.

The explosion of mobile devices and content, server virtualization, and advent of cloud services are among the trends driving the networking industry to re-examine traditional network architectures.[22] Many conventional networks are hierarchical, built with tiers of Ethernet switches arranged in a tree structure. This design made sense when client-server computing was dominant, but such a static architecture is ill-suited to the dynamic computing and storage needs of today's enterprise data centers, campuses, and carrier environments. Some of the key computing trends driving the need for a new network paradigm include:

Changing traffic patterns

Within the enterprise data center, traffic patterns have changed significantly. In contrast to client-server applications where the bulk of the communication occurs between one client and one server, today's applications access different databases and servers, creating a flurry of "east-west" machine-to-machine traffic before returning data to the end user device in the classic "north-south" traffic pattern. At the same time, users are changing network traffic patterns as they push for access to corporate content and applications from any type of device (including their own), connecting from anywhere, at any time. Finally, many enterprise data centers managers are contemplating a utility computing model, which might include a private cloud, public cloud, or some mix of both, resulting in additional traffic across the wide area network.

The "consumerization of IT"

Users are increasingly employing mobile personal devices such as smartphones, tablets, and notebooks to access the corporate network. IT is under pressure to accommodate these personal devices in a fine-grained manner while protecting corporate data and intellectual property and meeting compliance mandates.

The rise of cloud services

Enterprises have enthusiastically embraced both public and private cloud services, resulting in unprecedented growth of these services. Enterprise business units now want the agility to access applications, infrastructure, and other IT resources on demand and à la carte. To add to the complexity, IT's planning for cloud services must be done in an environment of increased security, compliance, and auditing requirements, along with business reorganizations, consolidations, and mergers that can change assumptions overnight. Providing self-service provisioning, whether in a private or public cloud, requires elastic scaling of computing, storage, and network resources, ideally from a common viewpoint and with a common suite of tools.

"Big data" means more bandwidth

Handling today's "big data" or mega datasets requires massive parallel processing on thousands of servers, all of which need direct connections to each other. The rise of mega datasets is fueling a constant demand for additional network capacity in the data center. Operators of hyperscale data center networks face the daunting task of scaling the network to previously unimaginable size, maintaining any-to-any connectivity without going broke.

The following list defines and explains the architectural components:[23]

SDN Application

SDN Applications are programs that explicitly, directly, and programmatically communicate their network requirements and desired network behavior to the SDN Controller via a northbound interface (NBI). In addition they may consume an abstracted view of the network for their internal decision making purposes. An SDN Application consists of one SDN Application Logic and one or more NBI Drivers. SDN Applications may themselves expose another layer of abstracted network control, thus offering one or more higher-level NBIs through respective NBI agents.

SDN Controller

The SDN Controller is a logically centralized entity in charge of (i) translating the requirements from the SDN Application layer down to the SDN Datapaths and (ii) providing the SDN Applications with an abstract view of the network (which may include statistics and events). An SDN Controller consists of one or more NBI Agents, the SDN Control Logic, and the Control to Data-Plane Interface (CDPI) driver. Definition as a logically centralized entity neither prescribes nor precludes implementation details such as the federation of multiple controllers, the hierarchical connection of controllers, communication interfaces between controllers, nor virtualization or slicing of network resources.

SDN Datapath

The SDN Datapath is a logical network device that exposes visibility and uncontested control over its advertised forwarding and data processing capabilities. The logical representation may encompass all or a subset of the physical substrate resources. An SDN Datapath comprises a CDPI agent and a set of one or more traffic forwarding engines and zero or more traffic processing functions. These engines and functions may include simple forwarding between the datapath's external interfaces or internal traffic processing or termination functions. One or more SDN Datapaths may be contained in a single (physical) network element—an integrated physical combination of communications resources, managed as a unit. An SDN Datapath may also be defined across multiple physical network elements. This logical definition neither prescribes nor precludes implementation details such as the logical to physical mapping, management of shared physical resources, virtualization or slicing of the SDN Datapath, interoperability with non-SDN networking, nor the data processing functionality, which can include OSI layer 4-7 functions.

SDN Control to Data-Plane Interface (CDPI)

The SDN CDPI is the interface defined between an SDN Controller and an SDN Datapath, which provides at least (i) programmatic control of all forwarding operations, (ii) capabilities advertisement, (iii) statistics reporting, and (iv) event notification. One value of SDN lies in the expectation that the CDPI is implemented in an open, vendor-neutral and interoperable way.

SDN Northbound Interfaces (NBI)

SDN NBIs are interfaces between SDN Applications and SDN Controllers and typically provide abstract network views and enable direct expression of network behavior and requirements. This may occur at any level of abstraction (latitude) and across different sets of functionality (longitude). One value of SDN lies in the expectation that these interfaces are implemented in an open, vendor-neutral and interoperable way.

The implementation of the SDN control plane can follow a centralized, hierarchical, or decentralized design. Initial SDN control plane proposals focused on a centralized solution, where a single control entity has a global view of the network. While this simplifies the implementation of the control logic, it has scalability limitations as the size and dynamics of the network increase. To overcome these limitations, several approaches have been proposed in the literature that fall into two categories, hierarchical and fully distributed approaches. In hierarchical solutions,[24][25] distributed controllers operate on a partitioned network view, while decisions that require network-wide knowledge are taken by a logically centralized root controller. In distributed approaches,[26][27] controllers operate on their local view or they may exchange synchronization messages to enhance their knowledge. Distributed solutions are more suitable for supporting adaptive SDN applications.

Controller Placement

A key issue when designing a distributed SDN control plane is to decide on the number and placement of control entities. An important parameter to consider while doing so is the propagation delay between the controllers and the network devices,[28] especially in the context of large networks. Other objectives that have been considered involve control path reliability,[29] fault tolerance,[30] and application requirements.[31]

OpenFlow uses TCAM tables to route packet sequences (flows). If flows arrive at a switch, a flow table lookup is performed. Depending on the flow table implementation this is done in a software flow table if a vSwitch is used or in an ASIC if it's implemented in hardware. In the case when no matching flow is found a request to the controller for further instructions is sent. This is handled in one of three different modes. In reactive mode the controller acts after these requests and creates and installs a rule in the flow table for the corresponding packet if necessary. In proactive mode the controller populates flow table entries for all possible traffic matches possible for this switch in advance. This mode can be compared with typical routing table entries today, where all static entries are installed ahead of time. Following this no request is sent to the controller since all incoming flows will find a matching entry. A major advantage in proactive mode is that all packets are forwarded in line rate (considering all flow table entries in TCAM) and no delay is added. The third mode, hybrid mode, follows the flexibility of a reactive mode for a set of traffic and the low-latency forwarding (proactive mode) for the rest of the traffic.

An SD-WAN is a Wide Area Network (WAN) managed using the principles of software-defined networking.[37] The main driver of SD-WAN is to lower WAN costs using less expensive leased lines, as an alternative or partial replacement of more expensive MPLS lines. Control and management is separated from the hardware, with central controllers allowing easier configuration and administration.[38]

A SD-LAN is a Local area network (LAN) built around the principles of software-defined networking, though there are key differences in topology, network security, application visibility and control, management and quality of service.[39] SD-LAN decouples control management, and data planes to enable a policy driven architecture for wired and wireless LANs. SD-LANs are characterized by their use of a cloud management system and wireless connectivity without the presence of a physical controller.[40]

SDN architecture may enable, facilitate or enhance network-related security applications due to the controller's central view of the network, and its capacity to reprogram the data plane at any time. While security of SDN architecture itself remains an open question that has already been studied a couple of times in the research community,[41][42][43] the following paragraphs only focus on the security applications made possible or revisited using SDN.

Several research works on SDN have already investigated security applications built upon the SDN controller, with different aims in mind. Distributed Denial of Service (DDoS) detection and mitigation,[44][45] as well as botnet[46] and worm propagation,[47] are some concrete use-cases of such applications: basically, the idea consists in periodically collecting network statistics from the forwarding plane of the network in a standardized manner (e.g. using Openflow), and then apply classification algorithms on those statistics in order to detect any network anomalies. If an anomaly is detected, the application instructs the controller how to reprogram the data plane in order to mitigate it.

Another kind of security application leverages the SDN controller by implementing some moving target defense (MTD) algorithms. MTD algorithms are typically used to make any attack on a given system or network more difficult than usual by periodically hiding or changing key properties of that system or network. In traditional networks, implementing MTD algorithms is not a trivial task since it is difficult to build a central authority able of determining - for each part of the system to be protected - which key properties are hid or changed. In an SDN network, such tasks become more straightforward thanks to the centrality of the controller. One application can for example periodically assign virtual IPs to hosts within the network, and the mapping virtual IP/real IP is then performed by the controller.[48] Another application can simulate some fake opened/closed/filtered ports on random hosts in the network in order to add significant noise during reconnaissance phase (e.g. scanning) performed by an attacker.[49]

Additional value regarding security in SDN enabled networks can also be gained using FlowVisor[50] and FlowChecker[51] respectively. The former tries to use a single hardware forwarding plane sharing multiple separated logical networks. Following this approach the same hardware resources can be used for production and development purposes as well as separating monitoring, configuration and internet traffic, where each scenario can have its own logical topology which is called slice. In conjunction with this approach FlowChecker[50] realizes the validation of new OpenFlow rules that are deployed by users using their own slice.

SDN controller applications are mostly deployed in large-scale scenarios, which requires comprehensive checks of possible programming errors. A system to do this called NICE was described in 2012.[52] Introducing an overarching security architecture requires a comprehensive and protracted approach to SDN. Since it was introduced, designers are looking at possible ways to secure SDN that do not compromise scalability. One architecture called SN-SECA (SDN+NFV) Security Architecture.[53]